Hydrodynamic
and mass transfer properties of a bubble column with vertically inserted tube
bundles

Nives ?imić^*, Kevin Breiler, Markus Schubert

Institute of Fluid Dynamics,
Helmholtz-Zentrum Dresden-Rossendorf, 01314 Dresden,
Germany

(^*corresponding
author?s e-mail address: n.simic@hzdr.de)

Keywords: bubble
column, internals, heat exchanger, tube bundle, hydrodynamics, gas holdup,
bubble size distribution, mass transfer, X-ray tomography

Introduction

Bubble columns are
widely utilized as 2- and 3-phase reactors and contactors in various industries
ranging from the chemical industry, food processing and metallurgy to
biotechnology. Due to the highly exothermic nature of a number of chemical
reactions conducted in these devices, the effective removal of the accumulated
reaction heat is highly important to ensure operational safety and to keep the
desired temperature constant. External and internal heat exchangers in a
variety of types (jacket heat exchangers, helical coils, single tubes and
bundles of tubes in different arrangements, tube diameters and numbers) are
utilized for this purpose. According to Saxena and Vadivel (1988) heat exchangers shaped
as helical coils and vertical or horizontal tube bundles present the most efficient ways to achieve favorable heat transfer rates.

The hydrodynamics, heat
and mass transfer rates of empty (without internals) bubble columns have been
studied, both experimentally and numerically, in great detail over a number of
decades. However, studies
examining the influence of vertically inserted tube bundles on the two-phase
flow conditions inside a bubble column and consequently on the heat and mass
transfer phenomena, the rates of which are dependent on the hydrodynamics, are
still scarce and lack proper understanding. Thus far
it is obvious that the hydrodynamic and transport properties of bubble
columns depend on the design of the tube bundle, namely on the diameter of the
tubes, the distance between the tubes, both of which determine the pitch, the
number of tubes and their configuration and ultimately on the percentage of the
cross-sectional area of the column covered by the bundle.

Although conflicting opinions about
the influence of tube bundles on the bubble and liquid dynamics exist and
though some intuitively unexpected results were obtained, it can be concluded
that their presence, depending on the superficial gas velocity and the tube
bundle design, can have a beneficial effect on the performance of bubble columns.
They are known to enhance bubble breakup (even in very viscous liquids), which
leads to a more uniform bubble size distribution and, hence, to increased
values of the overall gas holdup and specific interfacial areas (O´Dowd et al., 1987; Saxena and Vadivel,
1988; Chen et al., 1999). They are also
known to induce higher heat transfer rates than those encountered in empty
bubble columns (Abdulmohsin
and Al-Dahhan, 2012). On the downside, liquid
backmixing is enhanced by their insertion (Forret et al., 2003). Mass transfer rates have only
been reported for columns equipped with helical coils (Nosier and Mohamed, 2004).

Aim

 The objective of this study is to examine the influence of different
vertical tube bundle designs on the bubble dynamics and on the mass transfer
rates in a bubble column. The
studies in the open literature examining the performance of bubble columns with
vertically inserted tube bundles have focused primarily on the coverage of the
cross-sectional area of the bubble column by the tube bundle (CSA). The most
frequently used coverages are the 5% and the 25% (± 3%)
which mimic the heat exchangers utilized in the processes of methanol and
Fischer-Tropsch syntheses. Other than that, the designs of tube bundles seem to
be arbitrarily chosen and feature a number of different configurations of
layouts, tube diameters and tube lengths. From the current state of research,
it is thus rather difficult to draw conclusions on the optimal design of a heat
exchanger suitable for use in bubble columns (Youssef et al., 2013). Intuitively, it can be concluded that the most important design features of
tube bundles affecting the flow are the distance between the tubes and the unit
cell area enclosed by the tubes in their respective arrangements. Accordingly,
the study aims on a systematic analysis on the effect of these geometric
parameters.

Experimental methods

The experiments are
conducted in a 10-cm bubble column equipped with a perforated plate gas
distributor in the air-water system. Four tube bundle designs have been chosen and arranged in the triangular
and square pattern layouts, which represent the two most widely used heat
exchanger designs according to TEMA (Tubular Exchange Manufacturer´s
Association) and are known to affect the fluid turbulence to different extents (Mukherjee, 1998). A description of the
characteristics of the designs is presented in Table 1.

Table 1: Tube bundle designs and
characteristics.

The volumetric mass
transfer coefficient, k_La, is measured by
the oxygen absorption method using a commercially available oxygen probe. The
gas phase dynamics are obtained with the use of the in-house developed dual-plane ultrafast electron beam X-ray
tomography. The values of the superficial gas velocities are set to include
the three most frequently encountered flow regimes: the homogeneous, transition
and the heterogeneous regime. Mass transfer coefficients and gas phase dynamics
are determined at two axial positions encompassing both the sparger region (H/D
< 5) and the fully developed flow region (H/D > 5).

Results

The X-ray tomography is
a powerful non-intrusive technique, which enables imaging of two-phase bubbly
flows, namely the axial, radial and fractional gas holdups, bubble shapes, size
distributions, velocities, radial motions and interfacial areas, which have a
profound influence on the gas-liquid mass transfer.

In the presentation,
the effect of the internals designs on hydrodynamic and mass transfer will be discussed in several ways: a) on the basis of the pitch (or the
distance between adjacent tubes), b) the area of a unit cell (quasi-triangle
and quasi-square) enclosed by the tubes in the triangular and the square
patterns, respectively, c) the tube diameter and d) the coverage of the
cross-sectional area by the bundle.

The hydrodynamic and
mass transfer results are further compared with those of an empty (without internals)
bubble column of the same configuration (height and diameter) and under the
same operating conditions.

a)	b)

                                                                                                                                                                            
                          

Figure 1: a)
Schematics of the ultrafast X-ray tomograph, b) Example of a reconstructed image of the bubble column
cross-section obtained with the X-ray tomograph and the post-processed image
depicting bubbly flow.

For each of the patterns, mass transfer coefficients are measured and
coupled with the bubble dynamics.

References

1.      
Saxena S. C. and R.
Vadivel, Heat transfer from a tube bundle in a bubble column, Int. Comm. Heat Mass Transfer, 15, 657-667 (1988).

2.       O'Dowd W., D. N. Smith, J. A. Ruether and S. C.
Saxena, Gas and solids behavior in a baffled and unbaffled
slurry bubble column, AIChE J., 33,
1959-1970 (1987).

3.       Chen J., F. Li, S. Degaleesan, P. Gupta, M. H.
Al-Dahhan, M. P. Dudukovic and B. A. Toseland, Fluid
dynamic parameters in bubble columns with internals, Chem. Eng. Sci., 54,
2187?2197 (1999).

4.       Abdulmohsin R. S. and M. H.
Al-Dahhan, Impact of internals on the heat-transfer coefficient in a bubble
column, Ind. Eng. Chem. Res., 51, 2874-2881 (2012).

5.       Forret A., J. M.
Schweitzer, T. Gauthier, R. Krishna and D. Schweich,
Liquid dispersion in large diameter bubble columns, with and without internals,
Can. J. Chem. Eng., 81, 360?66 (2003)

6.       Nosier S. A. and M. M. Mohamed, Mass transfer at helical coils in bubble
columns, Chem. Biochem.
Eng. Q., 18, 235-239 (2004).

7.       Youssef A. A., M. H. Al-Dahhan and M. P. Dudukovic, Bubble columns with
internals: A review, Int. J. Chem.
Reactor Eng., 11, 1-55 (2013).

8.      
Mukherjee R.,
Effectively design shell-and-tube heat exchangers, Chem. Eng. Prog., 94, 21-37 (1998).