(18f) A Tomographic Study On the Effect of Liquid/Slurry Viscosity in a Slurry Bubble Column

A tomographic study on the effect of liquid/slurry
viscosity in a slurry bubble column

Swapna Rabha¹, Markus Schubert¹,
Uwe Hampel^1,2

¹Institute of
Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf, Germany

²AREVA
Endowed Chair of Imaging Techniques in Energy and Process Engineering, Dresden
University of Technology, 01062 Dresden, Germany

1.    
Introduction

Slurry bubble column
(SBC) reactors find a wide range of application in the chemical process,
pharmaceutical and biochemical industries, etc. Over the past decades, various
experimental and numerical studies have been done in an attempt to capture the
impact of solids in terms of solid concentration and solid particle size on
various hydrodynamic propertices^1-4. However, still the knowledge
about the possible effects of solids on the gas-liquid system is not understood
clearly. Due to this lack of clear understanding on the influence of the solid
particles, the prediction of the complex flow behavior of gas-liquid-solid
flows in slurry bubble column reactor becomes very difficult. In most of numerical
investigations^5-6, the gas flow behaviour was predicted considering
the uniform suspension of the solids in the liquid phase, i.e. the slurry phase
was modeled as a single pseudo-homogeneous phase. The assumption of a
pseudo-slurry phase may probably be reasonable for smaller particles where particle
Reynolds number, Re_P, is
below 0.3 and stokes law assumption is valid. However, for larger particles, where
Re_P is higher than 0.3,
the effect of the solid phase on the liquid cannot be neglected. Furthermore,
all the available correlations for slurry viscosity^7-8 consider only
the effect of C_s,
neglecting the effect of particle size and superficial gas velocity.

In the present work, an
attempt has been made to study the effect of viscosity in two-phase (air / water
+ glycerol) system and compared with the apparent (slurry) mixture viscosity in
three-phase (air / water +glass particles) system on the hydrodynamic parameters
like gas holdup at approximately same viscosities under similar operation
conditions.

2.    
Experimental setup
& ultrafast electron beam X-ray tomography

The experimental setup consists of a cylindrical column of 70 mm inner
diameter and 1500 mm height⁴. The details of the experimental setup are
explained by Rabha et al.⁴. The mixture
viscosity of the slurry (water + solid) was calculated from available
correlations as shown in Figure 1(a). The density and size of the glass
particles were 2500 kg.m^-1s^-1 and 100 µm, respectively. The
volumetric solid concentrations considered in the present work were 0, 0.05,
0.10, 0.20 and 0.36. On the other hand, the viscosity of the viscous liquid
(water + glycerol), which was almost equal to the mixture viscosity of the
slurry phase considered in the present work, was measured by a viscometer and is
shown in Table 1. The superficial gas velocities considered in the present work
were 0.02, 0.034, 0.05, 0.1 m/s.

In-house developed ?Ultrafast electron beam
X-ray tomography' was used to produce cross-sectional slice images of the flow
structure inside the slurry bubble column as shown in Figure 1(b). The details
of the experimental techniques, the applied tomography image reconstruction and
the applied procedure for calculating gas hold-up and approximate bubble size
distribution from the tomographic images are explained in previous publications^4,
9-11. For the present experimental studies, a temporal resolution of 2000
frames per second for 10 seconds of measurement was used.  The scan heights considered in the present
work were 300, 600 and 950 mm.

3.    
Results
and discussion

Experiments were conducted for liquid
viscosities (1 ≤ µ_L ≤  3.93 mPa.s)
at different gas superficial velocities (0.02 ≤ U_G ≤ 0.1 m/s) at different scan heights (H = 300,
600, 950 mm). The evolving
3D gas phase flow structures were visualized from a stack of 4000 reconstructed
cross-sectional tomographic images captured in 2 seconds (Figure 2). The
vertical coordinate of these virtual projections is the time. The time averaged
radial gas holdup was calculated from 20000 tomographic captured in 10 seconds
based on the grey level values.

3.1  Gas phase flow structure

The effect of viscosity (1
≤ µ_L ≤ 3.93 mPa.s) on the gas phase
flow structure for the two-phase (gas air / water + glycerol) system at U_G = 0.02 m/s and height H =
300 mm is shown in Figure 2 (a). The gas phase flow structures at different
solid loading (C_s = 0.05, 0.20,
0.36) which correspond to slurry viscosities of 1 ≤ µ_m≤ 4 mPa.s at U_G = 0.02 m/s and height H =
300 mm are shown in Figure 2(b).

With the increase of liquid viscosity, formation
of large bubbles was observed due to bubble coalescence. However, the effect of
apparent slurry viscosity on the bubble coalescence was more significant as
compared to that of liquid viscosities (Figure 2)

3.2  Radial gas holdup

The time-averaged
radial gas holdup profiles for the air / water + glycerol system (1
≤ µ_L ≤ 3.93
mPa.s) at gas superficial
velocity, U_G = 0.02 &
0.034 m/s and scan height H = 300 mm are plotted in Figure 3. The time-averaged
radial gas holdup profiles for air / water+ glass particles at mixture viscosity,
µ_m= 1.29 and 2.15 mPa.s which corresponds to solid concentration
C_s
= 0.05, 0.20 are also shown
in Figure 3.

The
effect of liquid & slurry viscosity on the time-averaged radial gas holdup profile
for both two-phase and three-phase system were insignificant at lower
viscosities (µ_L
= 1.33 and µ_m = 1.29 mPa.s). However, at higher viscosity, the
effect of slurry viscosity on the time-averaged
radial gas holdup was more pronounced as compared to that of the liquid
viscosity, which could be due to the interaction of the solid and the gas
phase.

4.    
Summary

Experimental investigations were done
for both two-phase and three-phase system to study the effect of liquid
viscosities (1 ≤ µ_L ≤ 3.92 mPa.s) and slurry viscosities
(1
≤ µ_m ≤ 4 mPa.s)
on the hydrodynamic parameters. The effect of slurry viscosity on the gas
holdup was significantly different than that of liquid viscosity at higher
viscosities which clearly shows the interaction of the particles on the gas
phase. The detailed results of the effect of viscosity (1 ≤ µ_m ≤ 4 mPa.s) on gas holdup and bubble size
distribution for both two phase and three phase system at different superficial
gas velocities (0.02 -0.1 m/s) and liquid heights (300 ? 950 mm) will be
discussed in the full length manuscript.

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