Investigation of gas–liquid dispersion and mass transfer performance of wide-viscosity-range impellers in water solutions of xanthan gum

Journal Pre-proof Investigation of gas–liquid dispersion and mass transfer performance of wide-viscosity-range impellers in water solutions of xanthan gum Baoqing Liu, Qing Xiao, Pengfei Gao, Bengt Sunden, Fangyi Fan

PII:

S0263-8762(19)30572-6

DOI:

https://doi.org/10.1016/j.cherd.2019.12.005

Reference:

CHERD 3928

To appear in:

Chemical Engineering Research and Design

Received Date:

10 June 2019

Revised Date:

13 November 2019

Accepted Date:

2 December 2019

Please cite this article as: Liu B, Xiao Q, Gao P, Sunden B, Fan F, Investigation of gas–liquid dispersion and mass transfer performance of wide-viscosity-range impellers in water solutions of xanthan gum, Chemical Engineering Research and Design (2019), doi: https://doi.org/10.1016/j.cherd.2019.12.005

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Investigation of gas-liquid dispersion and mass transfer performance of wide-viscosity-range impellers in water solutions of xanthan gum Baoqing Liu1,2,*, Qing Xiao1, Pengfei Gao1, Bengt Sunden2,*, Fangyi Fan1 (1. Institute of process equipment, Zhejiang University, Hangzhou 310027, China

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2. Department of Energy Sciences, Lund University, SE-22100 Lund,

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Sweden)

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Graphical abstract

Highlights

 LDB, FZ and MB impellers were studied in terms of gas dispersion and mass transfer.  Effects of gas flow rate, impeller speed and liquid concentration were

revealed.  Operating parameters and impellers under different conditions were recommended.

Abstract: Wide-viscosity-range impellers have extensive demands and applications in process industry. The gas-liquid dispersion and mass

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transfer characteristics of wide-viscosity-range impellers including Large-

double-blade (LDB) impeller, Fullzone (FZ) impeller and Maxblend (MB) impeller in water solutions of xanthan gum were investigated

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experimentally and compared. The influences of gas flow rate, impeller

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speed and polymer concentration of liquid on the power consumption, overall gas holdup εg and mass transfer coefficient KLa were also

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analyzed. On this basis, the appropriate operating parameters and impeller type were determined. The results indicate that with rising flow

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rate, the higher εg and KLa can be achieved with a drop in power

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consumption, and a relatively high flow rate is recommended on the premise of guaranteeing the complete dispersal condition in aerated

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vessel. Higher impeller speed provides better gas-liquid dispersion and mass transfer performance, but results in more power consumption simultaneously. The appropriate impeller speed should be just enough to meet the requirements of εg and KLa. It also is found that the increasing concentration of water solution of xanthan gum adds to the complexity of

gas dispersion and mass transfer in aerated vessel. Under the same specific power consumption PV, the εg and KLa are positive and negative correlation with the polymer concentration of liquid, respectively. Specially, when the concentration of water solution of xanthan gum is relatively low, the FZ impeller exhibits the best gas dispersion and mass transfer performance under the same specific power consumption.

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Nevertheless, the mass transfer performance of FZ impeller deteriorates significant when the concentration of water solution of xanthan gum

increases to 1.00 wt%, and the MB impeller becomes the most

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appropriate impeller type in this condition.

Keywords: wide-viscosity-range impeller; gas-liquid dispersion; mass

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transfer; non-Newtonian fluid

Nomenclature

Width of the flanged plate of the lower blade

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b

(mm)

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C

C*

Distance from tank bottom (mm) Saturated oxygen concentration in liquid phase (mg/L)

CL(t)

Oxygen concentration in liquid phase (mg/L)

C0

Initial oxygen concentration in liquid phase

(mg/L) Impeller diameter (mm)

Flg

Gas flow number

g

Gravitational acceleration (m/s2)

h

Space between upper and lower blades (mm)

h1

Height of the upper blade (mm)

h2

Height of the lower blade (mm)

h3

Height of the extension plate of the upper blade (mm)

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D

Liquid level height (mm)

Hg

Height of liquid after aeration (mm)

Ho

Height of liquid before aeration (mm)

ks

Constant

K

Consistency index (Pa·sn) Volumetric mass transfer coefficient (1/s) The number of grids

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m

Mi

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KLa

M

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H

Net torque (N·m) Idle torque (N·m)

Ml

Loading torque (N·m)

n

Flow behavior index

N

Rotating speed (r/s)

P

Rotating power (W)

Unaerated shaft power (W)

PV

Power per unit volume (W/m3)

Pg

Aerated shaft power (W)

Qg

Gas flow rate (m3/s)

Re

Reynolds number

RPD

Relative power demand

T

Vessel diameter (mm)

t

Time (s)

V

Volume of liquid phase (m3)

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Greek

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Po

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letters 

Angle between upper and lower blades (°)



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Radial angle between the flanged plate and lower blade (°)

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g

Global gas holdup (%)



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

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 avg

Shear rate (1/s)





Average shear rate (1/s) Apparent viscosity (Pa·s) Environmental temperature (°C) Density of water solution of xanthan gum (Kg/m3)



Shear stress (N/m2)

y

Yield stress (N/m2)

Abbreviations FZ

Fullzone impeller

LDB

Novel large-double-blade impeller

MB

Maxblend impeller

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1 Introduction

The system viscosity varies constantly in many situations of process

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industry. In some fermentation processes, for example, the viscosity of its

fermentation broth will increase gradually with the accumulation of

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products, which results in bubbles more difficult to disperse in stirred

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tank. (Garcia-Ochoa et al., 2000). Nevertheless, most of the studies on the mixing of xanthan gum were focused on single or multi-layer small-blade

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impellers (Funahashi et al., 1988; Sanchez et al., 1992; Xie et al., 2014; Li and Xu, 2016). It is difficult for these impellers to achieve good

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mixing and dispersion performance at all stages since there is a viscosity

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gradient in the non-Newtonian fluids. Accordingly, wide-viscosity-range impellers with better adaptability become urgent demands. There are two types of common wide-viscosity-range impellers, i.e.,

combined impeller and large-blade impeller. The combined impeller consists of multiple impellers driven by one or two independently rotating shafts. Compared with the single impeller, the combined impeller driven

by one rotating shaft can provide better gas dispersion and higher gas holdup, but it is not effective for highly viscous fluids. As the other combined impeller, the coaxial mixer consists of two independently driven impellers. The low-speed outer impeller of coaxial mixer is mainly responsible for cleaning up the vessel wall and bringing back the bulk fluids towards the centered impeller, while the high-speed inner impeller

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aims at producing intensive shear or circulation. The coaxial mixers are found to have excellent gas-liquid dispersion and mass transfer

performance in viscous fluids (Foucault et al., 2005; Bao et al., 2010; Liu

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et al., 2016a; Hashemi et al., 2016), and have the advantages of various

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impeller combinations and flexible operation patterns. However, its transmission system is relatively complicated and the dynamic sealing

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between inner and outer shafts is difficult.

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Besides the combined impeller, the other wide-viscosity-range impeller is the large-blade impeller, which accounts for a large proportion of the

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longitudinal section area of the stirred tank. The large-blade impeller can not only provide high mixing efficiency and large shear force, but also are

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competitive for their simple structure, easy dynamic seal and lower cost of operation and maintenance. As typical large-blade impeller, the Maxblend (MB) impeller, Full-zone (FZ) impeller and double helical ribbons (DHR) impeller are widely recognized in the process industry (Gu et al., 2000; Yamamoto et al., 1998). Zhang et al. (2012)

experimentally investigated the mixing and power characteristics of MB, FZ and Sanmeler (SM) impellers, and found that all the above impellers were suitable for processes with large viscosity changes. Yao et al. (2001) simulated the local mixing performance of MB and DHR impellers in a solid-liquid system. Results showed that the DHR impeller was able to induce a good total circulation throughout the stirring tank but unable to

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provide a promising local mixing performance, while the MB impeller has a good local dispersion performance, especially in the region with

grids. Dohi et al. (2004) investigated the solid suspension characteristics

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of MB and FZ impellers in a gas-solid-liquid system, and found that these

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impellers had good solid suspension performance, and the solid suspension performance of the MB impeller was better than that of the FZ

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impeller. Takahashi et al. (2005) measured the bubble size in a gas-liquid

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system agitated by an MB impeller, a Hi-F impeller and a dual Rushton turbine, respectively. As is found that large impellers are beneficial to

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improve the uniformity of the bubble size distribution. Chen et al. (2014) studied the structural features and performance advantages of MB and FZ

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impellers and then designed a new type of impeller called Large-doubleblade (LDB) impeller, which is the combination of the FZ and MB impellers. They investigated the power consumption and mixing characteristics of the novel LDB impeller in a single-phase system, and results showed that the mixing rate of the novel LDB impeller is faster

than that of a conventional impeller when the same power is consumed. Based on this research, (Liu et al., 2015, 2016b) measured and simulated the micro-mixing characteristics of a novel LDB impeller with the help of an iodide-iodate parallel competing reaction system. They found that whether the system viscosity is high or not, compared to FZ and DHR impellers, the novel LDB impeller has better micro-mixing characteristics

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with the same power consumption per unit volume. However, up to now,

most of researches about large-blade impeller were done separately, and

very few scholars have made a contrastive study of them in terms of the

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gas dispersion and mass transfer in gas-liquid two-phase systems.

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Moreover, most research works are done in Newtonian systems while non-Newtonian systems are more common in actual mixing operations.

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Thus, it is essential to make a comparative analysis on the gas-liquid

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dispersion and mass transfer characteristics of wide-viscosity-range impellers in non-Newtonian fluids.

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In order to determine the appropriate operating conditions and efficient impeller type for the gas-liquid dispersion and mass transfer in viscous

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non-Newtonian fluids, three wide-viscosity-range impellers including LDB, MB and FZ impellers were chosen and investigated experimentally in the two-phase system of water solution of xanthan gum and air. Furthermore, their performances at different gas flow rate, impeller speed and polymer concentration of liquid were also compared in terms of

power consumption, overall gas holdup and mass transfer coefficient.

2 Experimental setup and method 2.1 Experimental setup The experimental system for the research of gas-liquid dispersion and

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mass transfer includes aeration system, mixing system and measurement system (Liu et al., 2019). Specially, the aeration system consists of an air compressor, a gas sparger, a rotameter and a stable flow valve. The

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mixing system is composed of an electromotor, a stirred tank and an impeller. The measurement system includes a control cabinet, a dissolved

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oxygen electrode, a computer and some instruments. The specific

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information of relative measuring instruments is listed in Table 1. The stirred tank holding materials is made of transparent plexiglass and

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equipped with a standard ellipsoidal head, with internal diameter (T) of 380 mm. During the experiments, the gas-free liquid height in the tank is

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532 mm. That is, the ratio of height to diameter is 1.4, and the liquid

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volume is 56.7 L. Four baffles with uniform width of 32 mm are installed evenly on the internal wall of the stirred tank. The Large-double-blade (LDB) impeller, Fullzone impeller (FZ) and Maxblend (MB) impeller, which have wide applications in viscous systems, are used in the experiments. Structures of the three impellers are given in Figure 1, D is the impeller diameter, h is the axial distance of the two blades, α is the

radial angle between the upper and lower blades, n is the number of grids. Main size parameters of three impellers are listed in Table 2. The center of three impellers is 120 mm high above the tank bottom. The gas distributor installed at the bottom of stirred tank is often adopted to distribute the initial gas uniformly to achieve a better global dispersion effect. Liu et al. (2019) found that the diameter of bubbles

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dispersed by the novel spherical micro-orifice gas distributor is significantly smaller than that of the ring gas distributor under the same

gas flow rate, which leads to a higher gas residence time and gas holdup.

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It is good for improving the global gas-liquid dispersion and mass

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transfer characteristics. Thus, the novel spherical micro-orifice gas distributor was adopted in the experiment, the diameter of the micro-

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orifice is 0.22~100μm and the porosity is 30~40%. The novel spherical micro-orifice gas distributor with the diameter of 150 mm and the

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spherical height of 40 mm is installed on the tank axis with a distance of

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30 mm from the bottom (Liu et al., 2019).

2.2 Experimental material Water solutions of xanthan gum and air are selected as the liquid phase

and gas phase, respectively. In this research, three water solutions of xanthan gum at different polymer mass fractions (0.25%, 0.50%, and 1.00%) were obtained by dissolving xanthan gum powder in water.

Specially, the used xanthan gum powder is composed of D-glucose, Dmannose and D-glucuronic acid in the ratio of 3:3:2 as well as acetyl and pyruvic acid, and produced by Henan Huayue Chemical Products Co. Ltd. Water solution of xanthan gum is a shear thinning fluid and its rheological properties at three different polymer mass fractions were measured by an RS6000 rheometer (HAAKE, Germany) with a cone-

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plate sensor. Figure 2 shows the flow curves for shear rates from 0 to 600 1/s. The values of K and n of water solution of xanthan gum with

different concentrations were obtained by fitting a curve according to the

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power-law equation and are listed in Table 3.

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As a polysaccharide yielding shear-thinning, inelastic aqueous solutions, the rheology of water solution of xanthan gum can be described

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by Herschel-Bulkley model. Thus, its shear stress (τ) is given by Equation (1).

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   y  K n

(1)

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Viscosity of non-Newtonian fluids varies with shear rate, it is hard to determine the actual viscosity of fluids in stirred tanks because of the

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different shear rates at different locations. Thus, the apparent viscosity method proposed by Metzner and Otto (1957) is used widely, which assumes that the average shear rate in the stirred tank is correlated linearly with the impeller speed, viz:  avg  ks N

(2)

where ks is a proportional constant ranging from 11 to 25 according to the geometry of the impellers (Patel et al., 2012, 2013). For a HerschelBulkley fluid, the relation between average shear rate and apparent viscosity of the solution is as follows: 

  avg



 ks N



 y  K  ks N 

n

(3)

ks N

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Therefore, the Reynolds number is defined as: ks N 2 D 2   ND2 Re     y  K (ks N ) n

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(4)

2.3 Experimental method

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The impeller type, gas flow rate, stirring speed and polymer

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concentration of liquid are important factors to influence the gas-liquid dispersion and mass transfer characteristics of large-blade impellers in

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non-Newtonian fluids. Therefore, different experimental conditions were investigated with three impellers, in order to determine the effect of gas

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flow rate Qg, which was varied from 0 to 8.33x10-4m3/s, and of the

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impeller speed, N, which ranged from 1r/s up to 2.5r/s. As the impeller speed varies in the range of 1~2.5r/s, the value of apparent viscosity varies in the range of 42~528 mPa‧s and Reynolds number ranges from 135 to 4260. In high viscosity liquids, large diameter impellers are used at lower rotational speed. The first reason to cut down the impeller speed is to avoid mechanical damage. Moreover, high rotational speed is also

impractical in many biotechnological applications where materials are shear-sensitive and high shear rates always lead to the generation of byproducts (Lamberto et al., 1996). Moreover, with the help of the visual method, no flooding phenomenon was found in all of investigated conditions, and the specific measurement method is described below. (1) Power consumption

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During the experiment, the rotating speed (N) was adjusted by a

frequency converter, and the torque value (M) was measured by a torque

sensor. Because the friction losses are inevitable, the net torque (M) can

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be obtained by subtracting the idle torque (Mi) from the loading torque

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(Ml), as shown in Equation (5). Thus, the stirring power and the stirring power per unit volume were calculated according to the Equation (6) and

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Equation (7), respectively.

(5)

P  2 MN

(6)

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M  Ml  Mi

PV  P / V

(7)

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(2) Overall gas holdup

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The overall gas holdup in the stirred tank was calculated by measuring the liquid level difference. A ruler with a precision of 0.1mm was fixed on the outer wall of stirred tank for the measurement of liquid level before and after aerating. The ratio of the difference to original liquid level (Ho) was defined as the overall gas holdup, as shown in Equation (8):

g =

H g - Ho

(8)

Ho

(3) Volumetric mass transfer coefficient The volumetric mass transfer coefficient (KLa) was measured by using the dynamic oxygen electrode technique whose accuracy and reliability of the dynamic oxygen electrode technique where discussed carefully in previous paper (Liu et al., 2019). Nocentini et al. (1993) pointed out that

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the dissolved oxygen electrode should be placed in the middle vessel height, namely, a completely dispersed area, where the measured data

could better reflect the mass transfer coefficient of the entire vessel.

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Therefore, in the experiment, the dissolved oxygen electrode was placed

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in the middle of the vessel near the inner wall and the distance from the upper end of the electrode to the liquid level is 40mm.

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In the dynamic technique, the oxygen concentration of the inlet gas

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changes stepwise and the time-varying oxygen concentration in the liquid phase CL(t) is measured with an oxygen electrode. At the beginning of the

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experiment, air was pumped into the stirred tank until the dissolved oxygen concentration reached the saturation point, and then the saturated

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oxygen concentration C* at current temperature was logged. Then, the air compressor was turned off while the mixer was turn on, at the same time, nitrogen was pumped into the stirred tank to eliminate oxygen from the liquid. Finally, the air compressor was turned on until the CL(t) rises to 80%~90% of C*, and the dynamic change process of CL(t) over time was

recorded to get the response curve. There are two reasons to eliminate oxygen from the liquid before turn on the air compressor (Linek et al., 1996), one is to prevent the initial gas holdup in liquid from inhibiting the variety of the oxygen concentration at the early stage, the other is to avoid the interfere caused by the mixing of new and old gases. In addition, it is difficult to eliminate oxygen from the liquid completely, so it is only

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necessary to add nitrogen until the dissolved oxygen concentration had

declined to below 5% of the saturated oxygen concentration C*, which has been proved to have little effect on the results by some scholars

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(Chisti et al., 2002). Some scholars have found that the KLa values are

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basically the same as no matter what oxygen concentration in aeration gas is used and not affected by axial dispersion of both phases, which means

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that the dynamic variation of gases has negligible effect on the value of

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KLa (Linek et al., 1993; Moucha et al., 1998). Besides, in order to obtain an accurate result for curve fitting, the linear part of the response curve

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for oxygen concentration, which varies over time, that is, the range from 20% to 80% of the saturated oxygen concentration was selected. The KLa

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can be calculated by Equation (9). In our experiments, the gas phase is well mixed, so the C* could be assumed invariable throughout the vessel (Riet and Klaas, 1979). Thus, Equation (10) can be obtained by integrating Equation (9). dCL(t ) / dt  K L a  (C - CL(t ) )

(9)

ln

CL ( t ) - C * C0 - C *

 -K L a  t

(10)

Taking time as the abscissa and ln (CL(t ) - C* ) (C0 - C* ) as the ordinate, the KLa is actually the opposite of the straight line's slope. Considering that the saturated oxygen concentration is related to temperature, it is necessary to perform a temperature correction for KLa according to Equation (11) (Jackson and Shen, 2010; Moilanen et al., 2008). ( K L a) 1.022  20

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( K L a)20C 

(11)

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3 Results and Discussion 3.1 Influence of gas flow rate

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When the rotating speed is constant, the aerated shaft power (Pg) is less

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than the unaerated shaft power (Po). The ratio of Pg to Po is defined as the relative power demand (RPD). The relationship curve between the

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relative power demand (RPD) and the gas flow number Flg (Flg=Qg/ND3) is often used to assess the effect of gas flow rate on power consumption

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of mixing equipment.

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The effects of Flg on RPD of three wide-viscosity-range impellers are shown in Figure 3, and the polymer concentration of liquid and rotating speed were 0.50 wt% and 2r/s, respectively. As can be seen from Figure 3, the RPD of the three wide-viscosity-range impellers decreases significantly after the gas is pumped into the vessel. Generally, the apparent density of the fluid swept by the blades would be reduced after

aerating, which was considered as the main cause resulting in the decrease of stirring power. According to the cavity formation theory (Marshall and Andre Bakker, 2004; Riet and Smith, 1973, 1975), it is known that a low-pressure zone is formed at the back of the blades during the high-speed rotation of the impeller. Then the gas accumulates in the low-pressure regions and cavities form, which reduces the rotational

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resistance of the impeller, thus the shaft power drops significantly compared with the case of no ventilation. At the same time, it can be found that there exists significant negative and linear correlation between

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the value of RPD and the value of Flg when Flg is beyond 0.004. This is

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because the formation of big cavity is restrained effectively by the wideviscosity-range impeller, and thus the RPD decreases slowly, which is

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similar to the research results of Garcia-Ochoa and Gomez, (1998).

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By comparing the power curves of the three impellers, it can be found that the FZ impeller always has the lowest RPD. This is because the area

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swept by the FZ impeller is the largest, and the gas accumulated at the back of the blades is the most. Thus, the cavity is more likely to form, so

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the shaft power is much lower than that of no ventilation. However, for the other two wide-viscosity-range impellers, there is a reduction in the sweeping area because the grids are set on blades, which leads to less gas accumulate at the back of the blade and thus it is less likely to lead to cavity formation. Accordingly, the RPD of both is higher than that of FZ

impeller. The above analysis shows that the degree of power reduction before and after ventilation is positively related to the area swept by the blades. The influences of Flg on εg of the three wide-viscosity-range impellers are shown in Figure 4, and the polymer concentration of liquid and rotating speed were 0.50 wt% and 2r/s, respectively. As can be seen from

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Figure 4, the higher Flg is, the higher εg will be, and the rise rate of εg decrease with the increase of Flg. The reason for the increase of εg is that a larger gas flow rate results in more bubbles. However, the increase in

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number of bubbles leads to a greater possibility of bubble coalescence,

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which results in an increase in bubble size (Liu et al., 2019; Hashemi et al., 2015). The large bubbles overflow the stirred tank faster than small

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bubbles, which is not conducive to a further improvement of εg. Therefore,

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based on the above two reasons, the rise rate of εg decrease with the increase of Flg.

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At the same time, it can be seen that the εg of the three impellers is very close when Flg is small. The reason for this small difference in the εg is

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that the three impellers have similar structures and shaft power, and the gas in the vessel is completely dispersed when Flg is small. By comparing the three impellers, it can be found that the FZ impeller always has the highest εg. One reason is that there is no grid on the blades of the FZ impeller, thus the sweeping area is larger than that of other two impellers.

Another reason is that the FZ impeller has sweepbacks on the lower blade, which leads to a better discharge performance (Li et al., 2014), a larger circulation flow and a uniform dispersion of the gas. For these reasons, the overall gas holdup of the FZ impeller is higher than that of the others. The influence of Flg on KLa of the three wide-viscosity-range impellers is shown in Figure 5, and the polymer concentration of liquid and rotating

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speed were 0.50 wt% and 2r/s, respectively. Figure 5 shows that the KLa increases with increasing Flg. This is because the larger the gas flow rate,

the more bubbles in the vessel. Correspondingly, the εg in vessel and

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specific area between liquid and gas also increase, which is beneficial to

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improve the mass transfer rate (Bouaifi et al., 2001).

By comparing the KLa of the three wide-viscosity-range impellers, it is

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found that the FZ impeller has a stronger ability to dissolve oxygen than

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the other two impellers. First, this is related to its excellent drainage capacity. Thus there is a better gas circulation and a greater probability

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for bubble breakage, hence the εg and specific area are also larger than those of the other two impellers. Second, the initial size of the bubbles is

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small and the dispersion of bubbles is uniform because a microporous gas sparger is used in the experiments, which weakens the effect of the shearing action of LDB and MB impellers. Therefore, the KLa of the FZ impeller is higher than that of LDB and MB impellers under the condition of 0.50 wt% water solution of xanthan gum.

3.2 Influence of impeller speed The rotating speed has a great influence on the effect of gas-liquid mixing and is directly related to the shaft power. In order to compare the gas dispersion performance of different impellers conveniently, the influence of rotating speed on the overall gas holdup and oxygen mass transfer coefficient is studied by replacing the rotating speed with the

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stirring power per unit volume PV. In the experiments, the impeller speed

varies from 1r/s to 2.5r/s, and the corresponding PV of the FZ, LDB and

MB impellers varies from 42.1 to 642.4 W/m3, 43.7 to 604.4 W/m3 and

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36.7 to 595.3 W/m3 respectively.

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The influence of PV on εg with different wide-viscosity-range impellers is illustrated in Figure 6, and the polymer concentration of liquid and gas

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flow rate were 0.50 wt% and 2.78x10-4 m3/s, respectively. Figure 6 shows

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that the εg increases with increasing PV. One reason is that there is larger drainage volume and stronger gas circulation due to the increasing speed,

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and the other is owing to the smaller bubble size and longer residence time in the vessel because the shear rate increases. Moreover, it can be

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seen that the FZ impeller maintains a high εg in the 0.50wt% water solution of xanthan gum. This is because the FZ impeller has the advantage of good discharge performance, which results in the circulation of bubbles sufficient and the resident time longer. However, it also should note that the MB impeller possesses the lowest

εg. As mentioned above, because a microporous gas distributor is used in the experiment, the initial size of the bubbles is small and the dispersion of bubbles is relatively uniform, which weakens the effect of shearing action on the εg. Although the MB impeller has excellent shear performance, the impact on εg is weakened. Moreover, the discharge performance of the MB impeller is not good. Therefore, the εg is lower

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than that of the other two impellers.

The influences of PV on KLa by different wide-viscosity-range impellers are illustrated in Figure 7, and the polymer concentration of

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liquid and gas flow rate were 0.50 wt% and 2.78× 10-4m3/s, respectively.

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It can be observed that KLa increases with increasing PV. On the one hand, the increase of PV contributes to a larger drainage volume, a stronger gas

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circulation and a more uniform gas dispersion, which lead to an increase

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in the overall gas holdup. On the other hand, the enhancement in shearing action of blades results in a more drastic breakage and smaller size of

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bubble, and the specific area of small bubbles are larger than big bubbles (Felix and Gomez, 2004). Therefore, the increase of PV is beneficial to

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the increase of KLa, and the ability of mass transfer is enhanced. 3.3 Influence of polymer concentration of liquid The εg-PV curves with different polymer concentration of liquid (0.25 wt%, 0.50 wt%) of the water solution of xanthan gum are shown in Figure 8. In the experiments, the gas flow rate was 2.78× 10-4m3/s. As the

impeller speed varies in the range of 1~2.5r/s, the value of Reynolds number varies in the range of 960~4260 for the 0.25 wt% and 312~1628 for the 0.50 wt%. For the same impeller, it should be noted that the εg in the 0.50 wt% water solution of xanthan gum is always higher than that in the 0.25 wt%. In fact, under the same operating conditions, the increase of polymer concentration of liquid reduces not only the possibility of

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bubble coalescence but also the turbulent intensity in the stirred vessel (Orvalho et al., 2015). The circulation of fluid becomes slower and the time for the bubbles to flow out of the vessel becomes longer. At the same

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time, the increase of the viscous resistance also prolongs the residence

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time of the gas in the vessel, which results in the growth of εg. Note that the system viscosity increases with the rising polymer concentration of

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liquid at a fixed shear rate. However, when the viscosity is large enough,

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it is difficult for small bubbles to coalesce and discharge, causing some small bubbles to stay in the stirred tank for a long time, which result in an

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increase of εg. These factors work together and lead to a relatively high εg in the 0.50 wt% water solution of xanthan gum. However, for the small

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bubbles remaining in the stirred tank for a long time, their oxygen content is too low to achieve an effective mass transfer. The KLa-PV curves for different polymer concentration of liquid (0.25 wt%, 0.50 wt%, 1.00 wt%) of the water solution of xanthan gum are shown in Figure 9. In experiments, the gas flow rate was 2.78× 10-4m3/s.

As the impeller speed varies in the range of 1~2.5r/s, the value of Reynolds number varies in the range of 135~726 for the 1.00 wt%. By comparison, it is found that the KLa of the FZ impeller is always higher than that of LDB and MB impellers in 0.25 wt% and 0.50 wt% water solution of xanthan gum. This shows that the FZ impeller has superiority in mass transfer at low or middle concentration. The main reason is that

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the FZ impeller has a larger drainage volume and thus the gas circulation

is more sufficient and drastic. So there is a larger εg. On the other hand, at high concentration, the ability to dissolve oxygen drops significantly for

-p

FZ impeller but slightly for LDB and MB impellers. In such conditions,

re

the KLa of MB impeller is larger than that of the novel LDB and MB impellers, which might be related to the properties of the water solution

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of xanthan gum and the structure of the impeller. The higher the

na

concentration of water solution of xanthan gum is, the smaller the n and the greater the K will be. As a result, the turbulence intensity of fluid

ur

reduces significantly. Additionally, Sousa et al. (2007) studied the interaction between Taylor bubbles in stagnant non-Newtonian fluids.

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Results showed that the bubbles coalesce quickly when their initial distance is small, and the shear-thinning property of the fluid is able to accelerate the process. Zheng et al. (2006) found that the larger the mass fraction of water solution of xanthan gum is, the more obvious the shear thinning effect of the fluid is. Therefore, it is more likely that bubbles will

coalesce and form large bubbles in high concentration of water solution of xanthan gum. In 1.00 wt% water solution of xanthan gum, the advantage of the large drainage volume of the FZ impeller disappears and the shearing action also becomes worse. Thus, it is difficult to break up the bubbles so that they overflow from the stirred tank quickly after coalescing. The above reasons have eventually led to the decrease of

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overall gas holdup and the deterioration of the ability to dissolve oxygen. However, for LDB and MB impellers, although their discharge performances are weak, they still maintain good shearing performance at

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high concentration because the opening grids have the effect of shearing

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and thinning the fluids. As a result, the LDB and MB impellers have a high intensity of turbulence and a good effect of gas dispersion and gas-

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liquid mixing at high concentration. Furthermore, the power consumption

na

of these two impellers is lower than that of the FZ impeller at the same rotating speed. The above reasons show that the novel LDB and MB

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impellers have a stronger ability to dissolve oxygen at high concentration. In conclusion, the influence of polymer concentration of liquid on KLa of

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the three impellers follows the order of FZ > LDB > MB. Further study on the influence of polymer concentration of liquid on

KLa shows that under the same operating conditions, the larger the mass fraction of water solution of xanthan gum is, the smaller the KLa will be. It is also noted that the upward trend of KLa tends to be flat with the

increase of PV in high concentration of water solution of xanthan gum. This is because the viscous resistance increases while the speed of gas circulation decreases with the increasing mass fraction. The possibility of bubble coalescence increases, which results in an increase in bubble size and a decrease in KLa (Liu et al.,2019). In addition, the viscosity increases while the turbulent kinetic energy decreases with the increasing

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mass fraction, leading to an increase in the thickness of the retention layer. Accordingly, these factors reduce the volumetric mass transfer coefficient. 4 Conclusions

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In order to determine the appropriate operating conditions and efficient

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impeller type, the performances of LDB impeller, FZ impeller and MB impeller under different operating conditions were investigated in shear-

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thinning water solution of xanthan gums and compared in terms of power

na

consumption, overall gas holdup and mass transfer coefficient. As the adjustable parameters, the gas flow rate, impeller speed and

ur

polymer concentration of liquid are closely related to the flow number Flg, power input and liquid viscosity, and have direct effects on the gas

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dispersion and mass transfer in stirred tank. For the three wide-viscosityrange impellers, both εg and KLa increase with rising flow rate, while the RPD decreases significantly. Meanwhile, the change of RPD and εg become progressively less obvious when the gas flow rate increases to some extent. It should be noted that excessively high flow rate easily

leads to the over-loading and even gas flooding. Therefore, in order to obtain the better gas dispersion and mass transfer, the flow rate can be increased on the premise of guaranteeing the complete dispersal condition in aerated vessel. When the system properties are limited by the process and cannot be adjusted, the impeller speed is the most simple and direct way to improve

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the gas dispersion and mass transfer in aerated tank. The increase of impeller speed results in the more power input, correspondingly both the

εg and KLa rise. However, the growths of εg and KLa become smaller and

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smaller with increasing power input, especially in the systems with high

re

polymer concentration of liquid. Thus, low impeller speed is recommended under the condition of meeting the requirements of εg and

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KLa.

na

As a typical shear-shinning non-Newtonian fluid, the higher the concentration of water solution of xanthan gum, the greater its viscosity.

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Under the same specific power consumption PV, the εg and KLa are positive and negative correlation with the polymer concentration of liquid,

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respectively. Eventually, the high liquid viscosity results in the weak fluid circulation and low turbulent intensity, and the possibility of bubble coalescence decreases, which means the inactive bubble with small size remains longer in stirred tank. As a result, the εg increases with rising polymer concentration of liquid, while the KLa decreases with it.

Among the three wide-viscosity-range impellers, the FZ impeller exhibits the best gas dispersion and mass transfer performances, and can achieve the highest εg and KLa under the same specific power consumption, which is due to its excellent drainage capacity. Nevertheless, in the water solution of xanthan gum with high concentration, the shear action of impeller becomes more important than

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the drainage capacity. At this moment, the mass transfer performance of

FZ impeller deteriorates significant, and the MB impeller becomes the

most appropriate impeller type. In conclusion, the selection of wide-

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viscosity-range impellers for the gas-liquid dispersion and mass transfer

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particularity of stirred system.

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in the aerated tank must pay more attention to the complexity and

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Declaration of interests

The authors declare that they have no known competing financial

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interests or personal relationships that could have appeared to influence

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the work reported in this paper.

Acknowledgment This work was financially supported by the National Natural Science Foundation of China (21776246, 21978255) and the Fundamental Research Funds for the Central Universities of China (2019QNA4020).

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References Bao, Y., Yang, B., Xie, Y., Gao, Z.M., Gao, Z.D., Liu, T., Gao, X. H., 2010. Power demand and mixing performance of coaxial impellers in non-Newtonian fluids. J. Chem. Eng. Jpn. 44, 57-66. Bouaifi, M., Hebrard, G., Bastoul, D., Roustan, M., 2001. A comparative study of gas hold-up, bubble size, interfacial area and mass transfer

ro of

coefficients in stirred gas-liquid reactors and bubble columns. Chem. Eng. Process. 40, 97-111.

Chen. M.Q., Liu, J.L., Qian. L.Y., Zhang, Y.K., Liu, B.Q., 2014. Research

-p

and development of novel wide adaptability agitator. Chem. Eng. Mach.

re

41, 1-5.

Chisti, Y., Jaureguihaza, U.J., 2002. Oxygen transfer and mixing in

lP

mechanically agitated airlift bioreactors. Biochem. Eng. J. 10, 143-153.

na

Dohi, N., Takahashi, T., Minekawa, K., Kawase, Y., 2004. Power consumption and solid suspension performance of large-scale impellers in

114.

ur

gas–liquid–solid three-phase stirred tank reactors. Chem. Eng. J. 97, 103-

Jo

Felix, Garcia-Ochoa., Gomez, E., 2004. Theoretical prediction of gasliquid mass transfer coefficient, specific area and hold-up in sparged stirred tanks. Chem. Eng. Sci. 59, 2489-2501. Foucault, S., Gabriel Ascanio, A., Tanguy, P.A, 2005. Power Characteristics in Coaxial Mixing:  Newtonian and Non-Newtonian

Fluids. Ind. Eng. Chem. Res. 44, 5036-5043. Funahashi, H., Hirai, I.K., Yoshida, T., Taguchi, H., 1988. Mechanistic analysis of xanthan gum production in a stirred tank. J. Ferment. Technol. 66, 355-364. Garcia-Ochoa, F., Gómez Castro, E., Santos. V.E., 2000. Oxygen transfer and uptake rates during xanthan gum production. Enzyme Microb.

ro of

Technol. 27, 680-690.

Garcia-Ochoa, F., Gomez, E., 1998. Mass transfer coefficient in stirred

tank reactors for water solution of xanthan gums. Biochem. Eng. J. 1, 1-

-p

10.

re

Gu, X.P., Feng, L.F., Liu, Y., Wang. K., 2000. Study of three new agitators. China Synthetic Rubber Ind. 23, 344-347.

lP

Hashemi, N., Ein-Mozaffari, F., Upreti, S.R., Hwang, D.K., 2015.

na

Experimental investigation of the bubble behavior in an aerated coaxial mixing vessel through electrical resistance tomography (ERT). Chem.

ur

Eng. J. 289, 402-412.

Hashemi, N., Ein-Mozaffari, F., Upreti, S.R., Hwang, D.K., 2016.

Jo

Analysis of power consumption and gas holdup distribution for an aerated reactor equipped with a coaxial mixer: novel correlations for the gas flow number and gassed power. Chem. Eng. Sci. 151, 25-35. Jackson, M.L., Shen, C.C., 2010. Aeration and mixing in deep tank fermentation systems. AIChE J. 24, 63-71.

Lamberto, D.J., Muzzio, F.J., Swanson, P.D., Tonkovich. A.L., 1996. Using time-dependent RPM to enhance mixing in stirred vessels. Chem. Eng. Sci. 51, 733-741. Li, J.D., Su, H.J., Xu, S.A., 2014. Numerical simulation of mixing characteristics of Fullzone impeller. Chem. Eng. (Chinese). 42, 40-44. Li, L.C., Xu, B., 2016. Numerical simulation of flow field characteristics

ro of

in a gas-liquid-solid agitated tank. Korean J. Chem. Eng. 33, 2007-2017.

Linek, V., Beneš, P., Sinkule, J., Moucha, T., 1993. Non-ideal pressure step method for kLa measurement. Chem. Eng. Sci. 48, 1593-1599.

-p

Linek, V., Moucha, T., Sinkule, J., 1996. Gas-liquid mass transfer in

re

vessels stirred with multiple impellers - I. gas-liquid mass transfer characteristics in individual stages. Chem. Eng. Sci. 51, 3203-3212.

lP

Liu, B.Q., Huang, B.L., Zhang, Y.J., Liu, J.L., Jin, Z.J., 2016a. Numerical

na

study on gas dispersion characteristics of a coaxial impeller with viscous fluids. Taiwan Inst. Chem. E. 66, 54-61.

ur

Liu, B.Q., Wang, M.M., Liu, J.L., Qian, L.Y., Jin, Z.J., 2015. Experimental study on micromixing characteristics of novel large-double-

Jo

blade impeller. Chem. Eng. Sci. 123, 641-647. Liu, B.Q., Xiao, Q., Sun, N., Gao, P.F., Fan, F.Y., 2019. Effect of gas distributor on gas-liquid dispersion and mass transfer characteristics in stirred tank. Chem. Eng. Res. Des. 145, 314-322. Liu, B.Q., Zhang, Y.K., Zheng, Y.J., Huang, B.L., Chen, X.G., Jin, Z.J.,

2016b. Micromixing simulation of novel large-double-blade impeller. Taiwan Inst. Chem. E. 66, 62-69. Metzner, A.B., Otto, R.E., 1957. Agitation of non-Newtonian fluids. AIChE J. 3, 3-10. Moilanen, P., Laakkonen, M., Visuri, O., Alopaeus, V., Aittamaa, J., 2008. Modelling mass transfer in an aerated 0.2 m3 vessel agitated by Rushton,

ro of

Phasejet and Combijet impellers. Chem. Eng. J. 142, 95-108.

Moucha, T., Linek, V., Sinkule, J., 1998. Effect of liquid axial mixing on

Czech. Chem. Commun. 63, 2103-2113.

-p

local kLa values in individual stages of multiple-impeller vessel. Collect.

re

Nocentini, M., Fajner, D., Pasquali, G., Magelli, F., 1993. Gas-liquid mass transfer and holdup in vessels stirred with multiple Rushton turbines:

lP

water and water-glycerol solutions. Ind. Eng. Chem. Res. 32, 19-26.

na

Orvalho, S., Ruzicka, M.C., Olivieri, G., Marzocchella, A., 2015. Effect of bubble approach velocity and liquid viscosity. Chem. Eng. Sci. 134,

ur

205-216.

Patel, D., Ein-Mozaffari, F., Mehrvar, M., 2012. Improving the dynamic

Jo

performance of continuous-flow mixing of pseudoplastic fluids possessing yield stress using Maxblend impeller. Chem. Eng. Res. Des. 90, 514-523. Patel, D., Ein-Mozaffari, F., Mehrvar, M., 2013. Using tomography to characterize the mixing of non-newtonian fluids with a maxblend

impeller. Chem. Eng. Technol. 36, 687-695. Marshall, E.M., Andre Bakker, 2004. Handbook of Industrial Mixing: Science and Practice, in: Paul, E.L., Atiemo-Obeng, V.A., Kresta, S.M (eds), Computational Fluid Mixing. John Wiley & Sons, Inc., New York, pp. 385-386. Riet, K., Smith, J.M., 1973. The behaviour of gas -liquid mixtures near

ro of

Rushton turbine blades. Chem. Eng. Sci. 28, 1031-1037.

Riet, K., Smith, J.M., 1975. The trailing vortex system produced by Rushton turbine agitators. Chem. Eng. Sci. 30, 1093-1105.

-p

Riet, V., Klaas, 1979. Review of Measuring Methods and Results in

Process Des. Dev. 18, 357-364.

re

Nonviscous Gas-Liquid Mass Transfer in Stirred Vessels. Ind. Eng. Chem.

lP

Sanchez, A., Martinez, A., Torres, L., Galindo, E., 1992. Power

na

consumption of three impeller combinations in mixing Xanthan fermentation broths. Process Biochem. 27, 351-365.

ur

Sousa, R.G., Pinto, A.M.F.R., Campos, J.B.L.M., 2007. Interaction between Taylor bubbles rising in stagnant non-Newtonian fluids. Int. J.

Jo

Multiphase Flow. 33, 970-986. Takahashi, K., Tanimoto, I., Sekine, H., 2014. Gas dispersion in an agitated vessel equipped with large impeller. J. Chem. Eng. Jpn. 47, 717721. Xie, M.H., Xia, J.Y., Zhou, Z., Zhou, G.Z., Chu, J., Zhuang, Y.P., 2014.

Power consumption, local and average volumetric mass transfer coefficient in multiple-impeller stirred bioreactors for water solution of xanthan gums. Chem. Eng. Sci. 106, 144-156. Yamamoto, K., Abe, K., Tarumoto, A., Nishi, K., Kaminoyama, M., Kamiwano. M., 1998. Development and evaluation of large-Scale impeller generating strong circulation flow suitable for wide viscosity

ro of

range in reactor with cooling coil. J. Chem. Eng. Jpn. 31, 355-365.

Yao, W., Mishima, M., Takahashi, K., 2001. Numerical investigation on

ribbons. Chem. Eng. J. 84, 565-571.

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dispersive mixing characteristics of MAXBLEND and double helical

re

Zhang, H.Z., Yang, Z.W., Feng. B., 2004. Multifunctional large-sized wide-blade impeller. Chem. Eng. (Chinese). 32, 30-34.

lP

Zheng, Z.M., Yang, S.Z., Fan, Y.H., Yang, Q., Zen-Liang, Y.U., 2006,

na

Studies on fluid rheological properties and mass transfer during the fermentation of xanthomonas campestris in a stirred reactor. Nat. Gas

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Chem. Ind.(Chinese). 31, 27-31.

a

b )

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)

(

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(

Figure 1 Structures of three impellers

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(a) LDB impeller; (b) FZ impeller; (c) MB impeller.

0.1

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Viscosity(Pa·s)

na

1

0.25wt% 0.50wt% 1.00wt%

0.01

1

10

100

Shear Rate (1/s)

(a)

20

0.25wt% 0.50wt% 1.00wt%

Shear Stress(Pa)

15

10

5

0 100

200

300

400

500

Shear Rate (1/s)

(b)

600

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0

Figure 2 Flow curves for water solution of xanthan gum at three

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different polymer mass fractions

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(a) Rheograms of viscosity vs shear rate; (b) Rheograms of shear

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stress vs shear rate. 1.00

FZ LDB MB

na

0.98

RPD

0.96

0.94

ur

0.92

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0.90

0.88 0.000

0.004

0.008

0.012

0.016

0.020

0.024

Flg

Figure 3 Influence of gas flow number Flg on relative power demand RPD

0.095

FZ LDB MB

0.090 0.085 0.080

g

0.075 0.070 0.065 0.060 0.055 0.008

0.012

0.016

0.020

0.024

Flg

0.0056

FZ LDB MB

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0.0054

0.0052

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KLa(1/s)

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Figure 4 Influence of gas flow number Flg on overall gas holdup εg

0.0050

0.0046

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0.0048

0.008

0.012

0.016

0.020

0.024

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Flg

Figure 5 Influence of gas flow number Flg on volumetric mass

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transfer coefficient KLa.

0.08

FZ LDB MB

0.07

εg

0.06

0.05

0.04

0.03 0

100

200

300

400

500

600

700

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PV (W/m3)

Figure 6 Influence of unit volume power PV on overall gas holdup εg

0.007

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0.006

0.004

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KLa(1/s)

0.005

0.003 0.002

lP

0.001 0

100

200

300

400

500

FZ LDB MB

600

700

PV (W/m3)

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Figure 7 Influence of unit volume power PV on volumetric mass

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transfer coefficient KLa

0.08

0.07

0.06

0.25wt% 0.25wt% 0.25wt% 0.50wt% 0.50wt% 0.50wt%

0.04

0.03

0.02 0

100

200

300

400

500

600

700

PV (W/m3)

FZ LDB MB FZ LDB MB

800

900

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g

0.05

Figure 8 Influence of PV on overall gas holdup εg in different polymer

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concentration of liquid 0.016

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0.014 0.012

lP

KLa (1/s)

0.010 0.008 0.006

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0.004

0.25wt% FZ 0.25wt% LDB 0.25wt% MB 0.5wt% FZ 0.5wt% LDB 0.5wt% MB 1.0wt% FZ 1.0wt% LDB 1.0wt% MB

0.002 0.000

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0

100

200

300

400

500

600

700

800

900

PV (W/m3)

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Figure 9 Influence of PV on volumetric mass transfer coefficient KLa in different polymer concentration of liquid

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Table 1 Summary of the measuring instruments

DOG-3082

Electronic balance

LQ-A20002

Frequency converter

FR-D700

Graduated cylinder

250mL

Paperless recorder Rheometer

Shanghai Boqu Instrument Co., Ltd

±0.5%FS

Ruian Leqi Trading Co., Ltd.

0.01g

Mitsubishi

±5%

Kimble Bomex (Beijing) Glass Co., Ltd.

5mL

XSR90

Guangzhou noncon auto-control equipment co., ltd.

0.2%

RS6000

HAAKE, Germary

±1%

LZB-15

Yuyao Zhenxing Flowmeter Instrument Factory

±2.5%

TQ-660

Beijing Shitongkechuang Technology Co., Ltd.

0.1%

na l

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Torque sensor

Accuracy

pr

Dissolved oxygen electrode

Rotameter

Manufacturer

e-

Type

Pr

Instrument Name

Table 2 Main structural parameters of impellers m 6 0 8

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na

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Impelle h1(mm h2(mm h3(mm h(mm b(mm α β D r type (mm ) ) ) ) ) ,° ,° LDB 268 163 163 50 25 42 4 3 ) FZ 268 163 163 50 25 42 4 3 5 6 MB 268 314 77 / / / / / 5 6

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Table 3 Physical parameters of water solution of xanthan gum

0.483

0.50

2.462

1.00

6.54

0.374

0.194

997.8

0.205

1.486

995.9

5.156

991.0

0.16

na l Jo ur

 , kg/m3

pr

0.25

 y , Pa

n

e-

K, (Pa·sn)

Pr

Mass fraction, %