Hydrodynamics and mass transfer in an upflow monolith loop reactor

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Abstract

The hydrodynamics and mass transfer characteristics of a monolith loop reactor, with upflow of gas and liquid phases through the channels, have been investigated and compared with conventional internal airlift reactor and bubble column configurations. The volumetric mass transfer coefficient, kLa, is higher for monolith reactors than for airlift and bubble columns. This improvement is due to the superior mass transfer characteristics of Taylor flow in narrow capillaries. In order to gain further insight into the mass transfer from Taylor bubbles, experimental results are presented on the unit cell lengths and the Taylor bubble rise velocity.

Introduction

Monolith loop reactors are gaining considerable attention from academia and industry alike for carrying out solid catalysed gas–liquid reactions [1], [2], [3], [4]. Monolith loop reactors are being applied in the laboratory studies and in commercial practice for carrying out reactions such as hydrogenations [5], [6], [7], hydrodesulphurization [8], oxidations [9], bioremediation [10] and Fischer–Tropsch synthesis [11], [12]. Monolith reactors offer many potential advantages over trickle beds, slurry bubble columns and airlifts that include low pressure drop, high mass transfer rates, and ease of scale up [5], [13], [14]. Stankiewicz [14] provides an example of a process for which an in-line monolith reactor is 100 times smaller in size than a conventional reactor and therefore represents a truly intensified process. Most of the published experimental studies relate to downflow of both gas and liquid phases in monoliths [7], [13], [15], [16], [17], [18], [19] and very little information is available on upflow operation of gas and liquid [2]. In both upflow and downflow operations of monolith reactors we need to have uniform distribution of gas and liquid phases through the various channels.

The first major objective of the present work is to study the hydrodynamics and mass transfer in an upflow monolith loop reactor. The second objective is to develop the corresponding data on gas holdup, εG, and volumetric mass transfer coefficient, kLa, also for a bubble column and airlift, using the same column geometry and gas distribution device. In the study upflow monolith loop reactors of two different heights were employed. The data generated in this work would be helpful in choosing the right reactor type for any given process.

Section snippets

Monolith loop reactor

Experiments were performed in two upflow monolith loop reactor column configurations, schematically represented in Fig. 1. The setup shown on the left in Fig. 1 has a taller monolith bundle section (of height 2.1 m) than the setup shown in the inset, which has a monolith riser of height 0.65 m. For convenience, these setups would be referred to as tall and short monolith reactors, respectively. Hydrodynamic and mass transfer studies were carried out in both setups. The idea of the short monolith

Experimental procedure

Air was used as the gas phase and demineralized water as the liquid phase in all experiments carried out. Measurements were made of the gas holdup and volumetric mass transfer coefficient in each reactor configuration studied. Downcomer liquid velocity measurements were also made in the monolith and airlift loop reactors. At the start of each experimental run, the clear liquid height was set at 0.105 m in the gas–liquid separator of the short monolith loop reactor and 0.44 m in the gas–liquid

Monolith and airlift loop reactors

A common reactor model was developed for the monolith and airlift loop reactors for obtaining volumetric mass transfer coefficient values from experimental dynamic oxygen absorption curves based on the following assumptions:

  • Plug flow of gas and liquid in the riser section of the reactor. For the monolith reactors, tall and short, the minimum number of units cells is 11 and this rises to values in excess of 50. Therefore the assumption of plug flow is justified.

  • Plug flow of liquid in the

Results and discussions

The measured data on the gas holdup, εG, volumetric mass transfer coefficient, kLa, and the ratio kLa/εG, as a function of the superficial gas velocity UG based on the open area available for flow of the phases, for the reactor configurations investigated are summarized in Fig. 9a–c. The gas holdup in the airlift reactor is the lowest of the reactor configurations investigated. This is because of the high liquid circulation velocities in the airlift column (see Fig. 7); these high circulation

Conclusions

The hydrodynamics and mass transfer characteristics of monolith loop reactors, with upflow of gas and liquid phases through the channels, have been investigated in this study and compared with conventional internal airlift reactor and bubble column configurations. The following major conclusions can be drawn from this work.

  • (1)

    The volumetric mass transfer coefficient, kLa, is significantly higher for monolith reactors than for airlift and bubble columns. This improvement is due to the superior mass

Acknowledgements

Corning GmbH is gratefully acknowledged for provision of the cordierite square-channel monolith. We also acknowledge useful discussions with Dr. T. Boger of Corning. The Netherlands Foundation for Scientific Research—Chemical Sciences Division (NWO—CW), provides a research program subsidy. Sasol Technology Netherlands BV is gratefully acknowledged for partial financial support. We also thank a referee of this paper for pointing out the danger of saturation occurring in the tall monolith reactor.

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