14th European Conference on Mixing

Warszawa, 10-13 September 2012

MICROMIXING IN TWO- AND THREE-PHASE (S-L AND G-S-L)

SYSTEMS IN A STIRRED VESSEL

Julia Hofingera, Waldemar Bujalski

a, Serafim Bakalis

a, Archie Eaglesham

b and

Alvin W. Nienowa

a University of Birmingham, School of Chemical Engineering, Birmingham B15 2TT,

United Kingdom; b Huntsman Polyurethanes, Brussels, Belgium

A.W.Nienow@bham.ac.uk Abstract. The iodide/iodate reaction scheme was used to study micromixing in stirred

suspensions with and without gas sparging. The literature on the effect of added particles in turbulent stirred tanks is unclear and three-phase studies have not been reported before. In this work, a range of particle sizes did not affect the segregation index significantly at low concentrations (up to 7.2 wt.%). Higher concentrations, when cloud formation occurred, led to worse micromixing for feed near the surface, i.e. in the clear liquid layer, and near the impeller, i.e. in the cloud, which agrees with some earlier work. However, this was previously explained by turbulence modulation due to particles while here particle-particle interactions are also considered to be significant. For three-phases, small amounts (from 1.24 wt.%) of particles consistently reduced the previously reported [1] improvements in micromixing near the surface due to gassing; but there was still a significant enhancement compared to the ungassed case.

Keywords: micromixing, stirred tank, two-/three-phase systems, suspension, turbulence

modulation.

1. INTRODUCTION

Micromixing is important with fast competing chemical reactions which are often found in industrial processes such as the production of pharmaceuticals and fine chemicals. Therefore, micromixing has been studied extensively [2], especially in single-phase systems. In industrial applications, additional phases are frequently found in the reactor, for instance in crystallisation and chlorination, solid particles and gas bubbles are present, respectively. Unfortunately, there is much less data available on micromixing in two-phase systems and none for three-phase ones in spite of their relevance. However, it is well-established that micromixing and local turbulent energy dissipation rates, T , are closely related [2]. Therefore, information on the latter might indicate possible effects of adding solid or gaseous phases in chemical reactors or, at least, aid planning such micromixing tests.

The effect of particles on T in idealised flows suggests that small particles suppress it and larger ones augment it [3, 4]. For stirred tanks, there is less agreement on their impact with different methods such as micromixing studies and laser-based approaches giving different conclusions as reviewed previously [1]. In that work [1], results on the effect of gas bubbles (enhancing turbulence) and also of particles of one size over a wide range of concentrations were presented (no effect at low concentrations; dampening at higher concentrations with cloud formation). Here, new results for further particle sizes are presented comparable with those used in the turbulence modulation studies [3, 4]. Moreover, three-

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phase work, i.e. the combined effect of particles and gas bubbles, which has not been reported before, is included too.

2. EXPERIMENTAL

2.1 Iodide/Iodate method

The iodide/iodate reaction scheme [1] is based on two reactions: a neutralisation (1), which is quasi-instantaneous, and the Dushman reaction (2), which is a very fast redox reaction. These compete in parallel for H+ and, depending on micromixing, more or less of the side product, iodine from (2) is formed:

H2BO3− + H+ ⇌ H3BO3 (1)

5I− + IO3− + 6H+ ⇌ 3I2 + 3H2O (2)

I2 + I− ⇌ I3− (3)

Iodine then reacts to triiodide by quasi-instantaneous equilibrium (3) and can be measured spectrophotometrically. The measured extinction coefficient was was 2560.8 m2/mol at = 353 nm is in good agreement with literature [5]. The experimental procedure closely followed the literature [5, 6], first establishing that the experiments were conducted in the micromixing regime. The reactant concentrations are summarised in Table 1.

Table 1. Concentration of reactants (mol/L).

[𝐍𝐚𝐎𝐇] [𝐇𝟑𝐁𝐎𝟑] [𝐊𝐈] [𝐊𝐈𝐎𝟑] [𝐇𝟐𝐒𝐎𝟒] 0.0909 0.1818 0.0117 0.00233 0.5 or 1.0

The results of the experiments are expressed as the selectivity towards the unwanted

product or segregation index, 𝑋𝑆 = 𝑌/𝑌𝑆𝑇 , which is the ratio of actual yield of side-product, Y, to the maximum yield at total segregation, 𝑌𝑆𝑇 :

𝑌 =2 𝑛𝐼2

+ 𝑛𝐼3−

𝑛𝐻+0=

2𝑉𝑇𝑎𝑛𝑘 𝐼2 + 𝐼3−

𝑉𝐼𝑛𝑗𝑒𝑐𝑡𝑖𝑜𝑛 𝐻+ 0

(4)

and

𝑌𝑆𝑇 =6[𝐼𝑂3

−]

6 𝐼𝑂3− + 𝐻2𝐵𝑂3

−

(5)

Therefore, XS can range from 0 in the case of perfect micromixing to 1 for total

segregation. For the two-and three-phase results, in addition to XS itself, it was found useful to plot data as the deviation, XS from the single-phase or two-phase case respectively:

Δ𝑋𝑆 =𝑋𝑆 𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 − 𝑋𝑆 𝑣𝑎𝑙𝑢𝑒

𝑋𝑆 𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒

(6)

2.2 Experimental set-up

The equipment was the same as that reported previously [1]. A cylindrical Perspex tank, diameter T = 288 mm, with four baffles B = T/10 and a standard Rushton turbine, with a diameter D = T/3 was used with an impeller clearance, C = T/4 and an aspect ratio, H/T = 1.3. Power input was obtained by strain gauge telemetry and air was introduced through a ring sparger. After the reaction, samples were taken from a pipe in the side of the vessel, which was covered by a fine mesh to prevent solids being withdrawn. Two feed positions, one near the surface (pos. 1) and one near the impeller (pos. 2) as shown in Figure 1 were used, the coordinates of the feed points being specified in Table 2.

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Figure 1. Schematic of tank.

Table 2. Coordinates of feed pipe tip.

2r/D z/H

position 1 2.24 0.92 position 2 2.19 0.27

Glass Ballotini (dp = 150, 250, 500, 625 and 1125µm) and air were used as the solid and

gaseous phases, respectively. It has been verified before that both are suitable for use with the iodide/iodate reaction scheme and do not lead to unwanted reactions or losses of reactants [1].

2.3 Experimental conditions

For the low concentration solid suspensions, the impeller speed chosen was a compromise. Ideally, full particle suspension was required. On the other hand, too high impeller speeds would cause air entrainment which should be avoided for the solid-liquid experiments. Therefore, an impeller speed of 660 rpm was chosen. When the added phase affected power input, the speed was modified slightly so that power per total mass was kept constant, for instance when adding gas or larger amounts of solids.

3. RESULTS AND DISCUSSION

3.1 Effect of solids on micromixing in dilute suspensions without cloud formation [7]

Figure 2 shows a small effect on segregation index when feeding near the impeller for all particle sizes. The dashed line indicates the single-phase result for reference and no systematic difference can be seen as would be expected from turbulence modulation theory. For lower concentrations, the results are so close to the liquid-only value that they fall within the error bars. At the highest concentration, all points indicate slightly worse micromixing, which might be explained by four-way coupling ,i.e. some energy being dissipated by particle-particle interaction. In order to magnify these effects, Figure 3 shows XS as a percent of the single-phase reference value and confirms that the effect of particles up to about 2.5 wt.% is negligible. The downward trend at 7.2 wt.% is very consistent for all sizes, indicating some turbulence dampening due to the presence of particles.

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Figure 2. Effect of particle size in dilute

suspensions on the segregation index as a function of particle concentration – near

impeller.

Figure 3. Effect of particle size in dilute

suspensions at several particle concentrations – in percent of single-phase segregation index

– near impeller. Similar experiments were performed feeding near the surface (Figure 4). Here, because of

the lower local specific energy dissipation rates, the absolute values of XS are significantly higher than for the previous feed position. As before, there is no effect up to 2.5 wt.% glass beads and whilst the highest concentration gives slightly more side-product. These deviations are summarized in Figure 5 which shows comparable trends to Figure 3.

Figure 4. Effect of particle size in dilute

suspensions on the segregation index as a function of particle concentration – near surface.

Figure 5. Effect of particle size in dilute

suspensions at several particle concentrations – in percent of single-phase segregation index – near

surface.

These results are in agreement with one earlier work [8], but disagree with a more recent one [9] where enhanced micromixing was reported leading to a reduction in XS of up to 20%.

3.2 Effect of solids on micromixing in dense suspensions (with a cloud [7])

The previous work [1] for 500 µm particles at different specific power inputs consistently

showed worse micromixing. However, papers on turbulence modulation [3, 4] suggest larger particles should augment turbulence and consequently, micromixing. Therefore, here even larger particles (1125 µm) have been used to investigate this disparity just using one specific power of 1.14 W/kg.

Table 3 summarises the results when feeding into the cloud.

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Interestingly, the bigger particles lead to an even bigger reduction of micromixing efficiency, i. e. turbulence dampening. This directly contradicts the turbulence modulation literature and therefore, indicates that two-way coupling alone will not aid understanding of

Table 3. Effect of cloud on micromixing near impeller.

particle size XS

500 µm -7.7% 1125 µm -14.2%

such cases. On the other hand, it is in agreement with recent measurements by Gabriele et al. using PIV [10], though their discussion employing turbulence modulation theories to explain it does not seem appropriate. At these concentrations, turbulence dampening due to four-way coupling also seems probable [11]. Such a concept has previously been used to develop a model for particle abrasion in stirred vessels, the specific power input into the tank being considered to cause elastic deformation and particle abrasion, in addition to fluid motion [12].

Table 4 summarises the results for feeding above the cloud. In both cases, very significant turbulence dampening is indicated though in this case, it is less with the bigger particles. Observation of the structure of clouds and the fluid flow above it indicates that the bigger particles lead to a much less well-defined cloud than the smaller ones. Therefore, the flow above the cloud seems less quiescent and micromixing consequently better.

Table 4. Effect of cloud on micromixing above the cloud.

particle size XS

500 µm -32.3% 1125 µm -16.9%

These results emphasise the importance of considering particle concentration when

interpreting particle-turbulence interaction. Clearly, the flow structure overall was significantly affected by the clouds.

3.3 Effect of solids and gas bubbles on micromixing in sparged suspensions

Even though small amounts of particles did not significantly affect micromixing in the 2-phase solid-liquid system, with 3-phases compared to the 2-phase gas-liquid system they consistently increased the segregation index when feeding near the surface (Figure 6). This increase is approximately the same for all particle sizes as Figure 7 emphasises.

Figure 6. Segregation index as a function of

gassing rate in dilute suspension (1.24 wt.%) - feed near surface.

Figure 7. Effect of particle size and sparge rate on micromixing – in percent of gas-liquid segregation

index – feed near surface.

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Nevertheless, micromixing near the surface is still better than without gas sparging. Unfortunately, quantifying the change in the local specific energy dissipation rates is difficult: as the segregation index depends on the chemical concentrations and the reaction kinetics. If the latter are well established, micromixing models [2,5] can be used to obtain local rates. Unfortunately, for the iodide/iodate method, recently the chemical kinetics have been shown to be poorly understood [13].

4. CONCLUSIONS

The iodide/iodate reaction scheme was used to study micromixing in dilute and dense solid suspensions (when cloud formation occurs) alone and also with gas sparging. Up to 2.5 wt.% glass beads from 150 to 1125 µm do not affect micromixing near the surface or near

the impeller. There is, however, a small dampening effect at 7.2 wt.% which might be explained by particle-particle interactions. At higher concentrations, when clouds occur, micromixing was consistently worse than in the equivalent single-phase case. This, again, does not point towards turbulence modulation but more towards four-way coupling. The high segregation indices when feeding into the clear liquid above the cloud emphasise the importance of feed pipe positioning. For three-phases, particle concentrations, which did not affect micromixing without sparging, consistently increased the segregation index when feeding near the surface. Nevertheless, the indices are still lower, i.e. better micromixing, than in the equivalent single-phase case, so that gas sparging still improves micromixing significantly.

5. REFERENCES

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