Effect
of internals on fluid dynamic parameters in bubble column: A
comparative study

Dinesh V.Kalaga^1,
3,
Harish J.Pant²,
Sameer V.Dalvi³,

Jyeshtharaj B.
Joshi^1,
4,
Shantanu Roy^5*

1. Department of
   Chemical Engineering, Institute of Chemical Technology, Matunga,
   Mumbai400019, India.

2. Isotope
   Production and Applications Division, Bhabha Atomic Research Centre,
   Anushaktinagar, Mumbai400094, India.

3. Department of
   Chemical Engineering, Indian Institute of Technology-Gandhinagar,
   Gandhinagar382424, India.

4. Homi Bhabha
   National Institute, Anushaktinagar, Mumbai400094, India.

5. Department of
   Chemical Engineering, Indian Institute of Technology-Delhi, New
   Delhi110016, India.

*roys@chemical.iitd.ac.in

Keywords:
Radioactive
particle tracking, Bubble column with internals, turbulent parameters

Bubble column
reactors are widely employed for carrying out many processes many
such as oxidation, hydrogenation, Fischer-Trospch synthesis and
liquid-phase methanol synthesis in industry. Enormous amount of heat
is generated in these reactors which need to be removed. Therefore, a
number of vertical heat exchanger tubes are installed to maintain the
desired process temperature, and sometimes for controlling flow
structures and back mixing [1]. Apart from these applications in
chemical and petrochemical industry, similar reactors with
concentrically installed vertical tube bundles also find applications
as fuel rods or fuel bundles in nuclear reactors for power
generation. The nuclear fission process, which occurs in the fuel
containing rods, is highly exothermic in nature and the generated
heat is removed by convecting liquid coolant (or heavy water) around
the rods or tubes. Most often, in boiling water nuclear reactors,
removal of heat takes place during change of phase from liquid to
steam resulting in a complex two-phase flow pattern [2].

Although
hydrodynamics in bubble columns with vertical internals have been
addressed in some publications [3-5], the effect of superficial gas
velocity on fluid dynamic parameters over a wide range has never been
reported on a comparative basis.

The
radioactive particle tracking (RPT) technique is an established
non-intrusive technique used for mapping liquid/solid flows in
multiphase reactors [2,6]. The implementation of RPT mainly requires
a radioactive tracer particle, an array of NaI scintillation
detectors and a computer controlled data acquisition system (DAS)
capable of acquiring radiation intensity at a frequency of 50 Hz or
lesser. In the current study, a Scandium-46 (Sc-46) radioactive
tracer particle having activity of 400 Ci
was embedded in 1.5 mm hollow polypropylene bead (diameter: 1.5 mm)
and a suitable air gap was maintained to match the density of the
tracer particle with that of the density of water, hence rendering
the particle neutrally buoyant with respect to the liquid. The tracer
particle was introduced into the experimental column and allowed to
move freely with the flow and its motion is tracked using twelve
(2"Ã?2") NaI scintillation detectors placed around the
column at three different axial locations. The locations of the
tracer particle at successive sampling time periods were determined
using the pre-established distance count relationship (calibration).
The movement of the tracer particle with respect to time yields the
instantaneous local liquid velocity. The instantaneous velocities are
averaged over the duration of actual experiment (approx. 12 hours) to
obtain the mean liquid velocities. The fluctuating components were
calculated from the difference of mean and instantaneous liquid
velocities. The mathematical equations used for calculating the fluid
dynamic parameters have nicely been tabulated by Roy et al. [6].

The present work
attempts to quantify the effect of a wide range of superficial gas
velocity ranging from 0.014 to 0.265 m/s and vertical tube internals
(area covered by the internals is 9-23%) on the hydrodynamic
parameters in a bubble column. Experiments were performed in a
cylindrical bubble column of 0.12 m internal diameter and
height of 1.2 m (Figure 1). The internals were aluminium tubes having
diameter of 12 mm and height of 1.2 m. The internals were comprised
of one central rod having outer diameter of 36 mm and three
concentric set of tubes which were located at dimensionless radial
locations of 0.46, 0.66 and 0.86. Two spacers, made up of stainless
steel were used to hold the internals tightly at the top and bottom
ends. Three different configurations were used to study the effect of
internals, the top view of these configurations is shown in Figure 2.

The effect of
superficial gas velocity on the time averaged liquid velocity
profiles for bubble column equipped with internals (area covered by
internals are 9 and 13%, respectively) and without internals is
depicted in Figure 3. Results indicate that with an increase in the
superficial gas velocity, the liquid circulation becomes more intense
for all the cases. Further, installation of internals to the empty
bubble column (no internals) makes the velocity profiles
progressively steeper (as one compares the case of one vertical tube
and five vertical tubes). When a large number of internal tubes are
employed (14), the profile is very steep in the vicinity of an
internal tube, however the â??region of influenceâ? of adjacent
internal tubes overlap with each other eventually leading to a
complex flow pattern. This contribution will present a comparative
analysis of the flow patterns and axially averaged mean velocity
profiles in bubble column with internals (ranging from sparse to
dense). Information on â??turbulence quantitiesâ?, such as RMS
velocities and kinetic energy of fluctuations will be also be
presented for three internal configurations. The role of adding the
internal tube in modulating the fluid dynamic parameters of a bubble
column (no internals) will be brought out. The results may be, with
some extrapolation, used to make qualitative statements about the
evolution of velocity profiles as more internal tubes are added.

References:

1. Youssef,
   A.
   A., Al-Dahhan, M. H. and Dudukovic, M. P., 2013.Bubble
   Columns with Internals: A Review.Int.
   J. of Chem. Reactor Eng.11,
   1â??55.

2. Todreas,
   N. E. andKazimi, M. S.,Nuclear
   Systems,
   Taylor and Francis,USA.

3. Chen
   J, Li F, Degaleesan S, Gupta P, Al-Dahhan MH, Dudukovic MP, Toseland
   BA. Fluid dynamic parameters in bubble columns with internals. Chem.
   Eng. Sci. 1999; 54; 2187-2197.

4. Larachi
   F, Desvigne
   D, Donnat L, Schweich D. Simulating the effects of liquid
   circulation in bubble columns with internals. Chem. Eng. Sci. 2006;
   61; 4195-4206.

5. Ahmed
   AY, Al-Dahhan MH. Impact
   of Internals on the Gas Holdup and Bubble Properties of a Bubble
   Column. Ind. Eng. Chem. Res.2009;
   48;
   8007-8013.

6. Roy,
   S., Kemoun, A., Al-Dahhan, M. H. and Dudukovic, M. P.,
   2005.Experimental investigation of the hydrodynamics in a
   liquidâ??solid riser.AIChE Journal 51,
   802-835.

Figure
1:
Schematic of the experimental set-up (all dimensions are in mm).

Figure
2: Three
configuration of internals (Top view).

(A)

(B)

(C)

Figure 3. Effect
of superficial gas velocity on mean liquid velocity for (A) Bubble
column without internals (B) Bubble column with one centered vertical
tube (C) Bubble column five vertical tube internals (along the plane
passing through the internal).